Support standalone tool outputs in turn/start (#41002)

## What changed

- Add an optional `toolOutput` to `turn/start` for starting or steering a turn with a named function-call output instead of user input.
- Emit standalone outputs as `functionCallOutput` thread items, persist them in durable history, and include them in resumed threads.
- Validate that tool outputs have a name, are not combined with nonempty `input`, satisfy the text-size limit, and use supported image URLs.

## Testing

- Cover standalone tool-output notifications, model input, and resumed history in both legacy and paginated history modes.
- Cover the text-size limit for structured tool output.

GitOrigin-RevId: a1eeec7b9b7de67b1a1c5e829a54552b85ec118b

codex-rs/rollout/src/persistence_metrics.rs

@@ -261,6 +261,7 @@ fn rollout_item_type(item: &RolloutItem) -> String {
 fn turn_item_type(item: &TurnItem) -> &'static str {
     match item {
         TurnItem::UserMessage(_) => "user_message",
+        TurnItem::FunctionCallOutput(_) => "function_call_output",
         TurnItem::HookPrompt(_) => "hook_prompt",
         TurnItem::AgentMessage(_) => "agent_message",
         TurnItem::Plan(_) => "plan",

codex-rs/rollout/src/policy.rs

@@ -96,7 +96,9 @@ pub fn should_persist_event_msg(ev: &EventMsg, history_mode: ThreadHistoryMode)
             matches!(history_mode, ThreadHistoryMode::Paginated)
                 || matches!(
                     event.item,
-                    TurnItem::Plan(_) | TurnItem::Extension(ExtensionItem::Sleep(_))
+                    TurnItem::FunctionCallOutput(_)
+                        | TurnItem::Plan(_)
+                        | TurnItem::Extension(ExtensionItem::Sleep(_))
                 )
                 || matches!(
                     &event.item,